Abstract:
In the late 1910s and early 1920s a pair of scientists independently developed an equation that describes how a fluid rises in a capillary tube as a function of time. The formula—the Lucas-Washburn equation—was based on observable macroscopic properties such as surface tension, viscosity, and radius of the capillary tube. It predicted that the height a fluid rose to in a given amount of time to be proportional to the square root of that time. For example: if in one second a fluid rose one millimeter, then it would take four seconds for it to rise two millimeters. The work behind this equation depended on the materials having defined macroscopic properties. As science has moved into the realm of the very small, where materials may no longer be continuous, but discrete; many of the equations that hold true at large scales fail at these new small scales. This is often why nanotechnology has attracted so much attention, new properties that can be exploited by working on such small scales.

Recently, a real scientific controversy has been brewing over this relationship and whether it still holds true at very small length scales. Some groups have reported that the height a fluid scales rises proportionally slower than the time, and other say that the height rises in a linear fashion with time. While this may not seem like a big deal to many, it is to the applications that rely on capillary action to function. Capillary action is at the heart of many technologies, such as dry-wicking fabrics, adsorbent paper towels, or newer nano-scale lab-on-a-chip devices that are poised to revolutionize many areas of science. Without an accurate understanding of how fluids move at these small scales, these technologies face even larger hurdles than they already do. Laboratory experiments on nanoscale capillary action have proven inconclusive to date. To probe this phenomenon in much greater detail, a team of researchers from Germany and Bulgaria developed a computer simulation to study how fluids move into very small tubes.